The present invention relates to a frequency divider, and more particularly, to a programmable frequency divider.
A frequency divider is a very important component of a frequency synthesizer and is used for dividing a source signal to form a result signal. The frequency of the result signal is the frequency of the source signal being divided with a divisor. As is known in the art, a conventional frequency divider is composed of a plurality of cascaded dual mode dividing cells. Each cell of the frequency divider performs a dividing operation according a corresponding divisor signal. The division range depends on the number of cells of the frequency divider. The more cells that are used, the wider the division range. A ⅔ cell having two dividing modes, which are divide-by-two and divide-by-three modes, is a commonly used dividing cell. If the frequency divider is composed of N ⅔ cells, the division range of the frequency divider is all integers from 2N to 2N+1−1.
Employing more cells in the frequency divider can extend the division range; however, it deteriorates space efficiency for IC design. U.S. Pat. No. 5,349,622 discloses a programmable frequency divider with a prescaler and a programmable counter. Adjusting the counter value of the programmable counter extends the division range of the frequency divider. The complexity of circuit design and cost, however, are increased due to the utilization of the programmable counter.
In other applications, such as a fractional frequency divider in a frequency synthesizer, the programmed divisor repeatedly changes from a value of M to a value of M+1, then back to M in order to obtain a result signal at a fractional frequency. However, if the divisor changes, i.e. a set of divisor signals are loaded while one or more of the dividing cells is sensitive to the divisor signal, the cells sensitive to the divisor signals are enabled to perform divide-by-two or divide-by-three operations according to the corresponding divisor signals. Therefore, some cells of the programmable frequency divider operate depending upon the divisor signals and the other cells operate depending upon the original divisor signals. The effective division may thereby result an incorrect value, i.e. the effective division may be a value that is neither M nor M+1.